In general, a construction machine as a hydraulic excavator is provided with an engine using a gasoline, a light oil or the like as fuel, a hydraulic pump driven by the engine, a hydraulic motor driven by pressurized oil delivered from the hydraulic pump, and a hydraulic actuator of a hydraulic cylinder or the like. The hydraulic actuator is small in size and light in weight, and is capable of producing large output, which is widely used as an actuator for the construction machine.
On the other hand, there is known a hybrid-type hydraulic excavator using both an engine and a motor generator together (refer to Patent Document 1). In this hybrid-type hydraulic excavator, for example, the motor generator and a hydraulic pump are attached on an output shaft of the engine and an electricity storage device is provided to be electrically connected to the motor generator. The motor generator has an electric generator function of charging the electricity storage device with power generated by a driving force of the engine and an electric motor function of assisting in the engine by power running using power of the electricity storage device.
There is known a hybrid-type hydraulic excavator that assists in a start of an engine based upon an electric motor function of a motor generator at an engine starting time by a discharge of an electricity storage device to improve start-up performance of the engine. Patent Document 2 describes a hybrid-type hydraulic excavator configured to assist in an engine start through a battery-drive of a motor generator in a case where an engine does not start, for example, due to a factor of a low temperature or the like regardless of the intent of starting the engine.
Further, Patent Document 3 describes a hybrid-type hydraulic excavator of performing a charge of an electricity storage device by causing an operation of an engine to continue at an engine stop command and in a case where a state of charge (SOC) as a charge rate of the electricity storage device falls below a lower limit value necessary for the next engine start. The hybrid-type hydraulic excavator holds the SOC at the engine stop time to a value necessary for the next engine start, thus making it possible to certainly perform the next engine start.
Further, Patent Document 4 describes the configuration that in a case where an engine is in a start-up condition, a limit to the discharging of an electricity storage device is suppressed by a discharge power limiting device to secure power necessary for the engine start and increase frequency in which the engine can start.